PORTLAND, Ore. (AP) -- Investigators who have searched for six days for a missing 7-year-old Oregon boy without success are turning to experts from across the state to assist in the expanding effort.

Watch breaking news video from the Associated Press here The search has been slowed by the rough terrain surrounding the rural Skyline Elementary School, where 7-year-old Kyron Horman was last seen last Friday morning after his stepmother dropped him off at a science fair. Several storms have crossed the area, drenching searchers who called off their efforts on Friday night but have been working nearly 24 hours a day since. On Wednesday, an experienced search-and-rescue expert said time is a factor in the search. Minutes later, a trickle of raindrops segued into a deluge that blew through the area. "We have more resources coming," said Klamath County Sheriff Tim Evinger. "The clock is ticking on this search." The hunt has been slow going, as the volunteer teams checked a half-mile radius around the school during the weekend, then did so again Tuesday. Dressed in fluorescent-green vests and T-shirts, the volunteers ventured into the dense foliage that towers over every road and building in the area. Information on the search has been scarce. Multnomah County sheriff's Capt. Jason Gates said he's reticent to say much about the volunteers' efforts or any evidence recovered because it could interfere with the investigation. Horman family members have declined interview requests. Some have told The Associated Press they were asked by police not to speak. Kyron disappeared after a science fair he attended with his stepmother, who said she last saw him as he walked down a hallway toward his second-grade classroom wearing a "CSI" T-shirt and dark cargo pants. Authorities said the last reported sighting of Kyron was at about 9 a.m., but they have refused to say who made that sighting. The search began after the boy did not come home on the school bus and his stepmother called 911 at about 3:45 p.m. The primary agency in the search, the Multnomah County sheriff's office, has been emphatic in its efforts to portray the arrival of the additional searchers as a "natural progression" in the process. The office said it was making use of a state law passed in 2007, after authorities were criticized for the way they conducted the search for James Kim. The California went missing in Southern Oregon in 2006 and was ultimately found dead of exposure. An ensuing review of the search by the Oregon State Sheriffs' Association concluded that the effort was marked by crossed signals and several people trying to take charge of the search. The concerns led to legislation streamlining communications between search-and-rescue units. That legislation was what led to the new search teams that were scheduled to arrive between Wednesday night and Thursday morning and "get to work right away," Evinger said. Gates has been careful to say that his agency remains in charge of the investigation, despite the new search experts and the presence since Monday of the FBI. Evinger, who headed a task force that recommended changes that resulted in the 2007 law, said the law foresees that local police conduct the search before crews from other counties are brought in. "We pull out all the stops locally, initially," Evinger said. "It starts getting prolonged, you go statewide." Fliers of Kyron dot the convenience stores and restaurants in the hills and river valley for miles around the school. Tips initially poured in -- Gates said he had 1,200 called in by Monday -- and police have been asking for more. Some tips claim sightings of Kyron, which Gates said the search teams have checked out and found unsubstantiated. Most are called in from or refer to the Portland metro area, said Multnomah County sheriff's spokeswoman Mary Lindstrand, so the search has focused there. Some calls, however, came from as far away as Washington state. Sgt. Diana Olsen, search-and-rescue coordinator for the sheriff's office, said the volunteers are worn out but remain persistent. "They're tired, exhausted," Olsen said. "(But) they don't want to stop." The boy's family released a statement at a news conference Wednesday asking residents around the school to check and recheck their property, outbuildings and sheds for any sign of the missing 7-year-old. "There are a lot of rescuers out there," the family said in the statement. "Please don't stop."